Oscar nominee Saoirse Ronan ("Atonement") is joining the fantasy-drama, which also stars Christina Hendricks, Matt Smith ("Doctor Who"), Eva Mendes, and Ben Mendelsohn.
"Monstar" is describes as a noirish modern day fairytale set in a dying city, where a single mother named Billy (Hendricks) stumbles into a dark fantasy underworld after her teen son finds a hidden pathway leading to an underwater town. Ronan's role has yet to be revealed.
It marks the directorial debut from Gosling, who has starred in such recent films as "Gangster Squad" and "Drive" (with Hendricks) and will soon be seen in "The Place Beyond the Pines," which co-stars Mendes.
Ronan will soon be seen starring in "The Host," based on a book by "Twilight" author Stephenie Meyer.
She is currently shooting Wes Anderson's "The Grand Budapest Hotel," co-starring Ralph Fiennes, Jude Law and Bill Murray.